4S ON THE COMMON BRITTLE STAR. 
in the same manner, and haye (subsequently to their formation) 
been gradually augmented in dimensions by the deposition of 
substance on their margins. 
Gosse in his “Tenby” gives a very clear and ingenious idea as 
to the growth of similar organs in the Echinus; his opinion is, 
that the points or teeth seen in so many of the parts of these 
animals are increased by a deposit, and meeting, they coalesce, 
and thus extend the size of the plate or other part, at the same 
time forming the perforations so frequently met with. 
This would only hold when the plates or the like are surrounded 
by, or contiguous with, a substance capable of secreting and de- 
positing the lime. 
It now only remains for me to give a description of the figures 
accompanying this paper. 
Pirate I.—Fig. 1 gives a general view of a form of disc fre- 
quently met with; were the various appearances that may be 
noticed, figured, they would only tend to confuse. 
Fig. 2 shows a large specimen with the plates on the ray-joints, 
and gives a general idea of the gradual assumption of the pointed 
form of spines. This figure also shows the mouth, with the five 
converging teeth meeting in the centre, together with the arrange- 
ment of the irregular plates, round the margin of the disc. 
Fig. 3. Tip of moderately large ray, the claws of which have been 
rapidly formed before the spines, which are just appearing from 
the base of the claws. This appearance is frequently seen in 
slender specimens, and appears to be the result of a rapid longi- 
tudinal growth. 
Fig.4. Early form of flat fan-shaped spine, showing the assump- 
tion of features causing it to possess more of the character of a 
regular spine. 
Figs. 5 to 12. Successive stages of the hooks, showing the 
changes from hooks to spines. 
Figs. 15 and 14. Two three-pronged spines from the scale plates 
upon upper surface of disc; these cause the rough spinous pro- 
cesses seen in mature forms. 
Nore.—Figs 1, 3,4, 5, 6, 7,8, 9,10, 11 12, are all magnified 100 diameters. Fig. 2 is 
magnified 75 diameters. Figs. 13 and 14 are magnified 200 diameters. 
